Associative Array in C

Discussion in 'C Programming' started by chadsspameateremail@yahoo.com, Mar 3, 2008.

Guest

What I am trying to do is implement a associative array to lookup data
based on a key in C. This might simple to do in C++ or Perl but I'm
restricted to use C. The size of the data for the table isn't too
large but too large to structure the code as a clean chain of if/else
or similar logic. Also, the lookup key values can be large,
unpredictable and not well dispersed which doesn't suit itself to a
hash table.

OK so my current idea for implementing this is using an array of C
structures which gets initialized and contains all the relevant
information. The array will be a module global variable used in the C
file which needs this functionality. The array definition would look
something like the following:

So I have implemented some prototype code based on this and it does
work to do lookups. But what I was wondering is how portable is
this? Is this legal C as defined by the standard to initialize an
array of structures in this way? Is this going to work across
platforms? I hope you see my dilemma the code appears to work on my
system. But I don't know if it's guaranteed to work everywhere and how
portable the library functions I'm working on will be because of it?

Advertisements

writes:
> What I am trying to do is implement a associative array to lookup data
> based on a key in C. This might simple to do in C++ or Perl but I'm
> restricted to use C. The size of the data for the table isn't too
> large but too large to structure the code as a clean chain of if/else
> or similar logic. Also, the lookup key values can be large,
> unpredictable and not well dispersed which doesn't suit itself to a
> hash table.
Sorry you could use a hash table without troubles. I bet you'd like to
look uf the Error Code by number and so this would be the key for you
hash table....
>
> OK so my current idea for implementing this is using an array of C
> structures which gets initialized and contains all the relevant
> information. The array will be a module global variable used in the C
> file which needs this functionality. The array definition would look
> something like the following:
>
> struct ErrorLookupTable
> {
> long DeviceErrorCode;
> char* ErrorString;
> };
>
> #define TableSize 6
>
> static struct ErrorLookupTable MyTable[ TableSize ] =
> {
> { 45, "Not enough memory" },
> { -66, "File I/O error" },
> { -2565, "Device locked" },
> { 32727, "Device not found" },
> { -32727, "Device not found" },
> { 65534, "Unknown device error" }
>
//... LOTS more
surely not you have set TableSize to 6 ...

> Is this legal C as defined by the standard to initialize an
> array of structures in this way?
Yes
>Is this going to work across
> platforms?
Yes

This code works and is portable, but it can be improved:
1. Instead of using a macro for the size, leave the size open and instead
compute the eventual size:
struct foo array[] = {...};
size_t const array_size = (sizeof array)/(sizeof *array);
This has the advantage that you don't have to manually keep the macro and
the array's size in sync.
2. A string literal must not be modified, but it will still (compatibility
with older code) convert to a modifyable pointer. However, you should not
use that, so you should make 'ErrorString' a 'char const*' instead. Unless
you need to modify it, you should also make the whole array constant, so
the compiler tells you when you accidentally try to modify it.
3. If you sorted the array by the error code, you could use a faster binary
search, assuming the error code is the key value. I'd actually put in a
test somewhere like assert(array_is_sorted()); then so you won't forget
that after code changes.

On Mar 2, 8:24 pm, wrote:
> What I am trying to do is implement a associative array to lookup data
> based on a key in C. This might simple to do in C++ or Perl but I'm
> restricted to use C. The size of the data for the table isn't too
> large but too large to structure the code as a clean chain of if/else
> or similar logic. Also, the lookup key values can be large,
> unpredictable and not well dispersed which doesn't suit itself to a
> hash table.
>
> OK so my current idea for implementing this is using an array of C
> structures which gets initialized and contains all the relevant
> information. The array will be a module global variable used in the C
> file which needs this functionality. The array definition would look
> something like the following:
>
> struct ErrorLookupTable
> {
> long DeviceErrorCode;
> char* ErrorString;
> };
>
> #define TableSize 6
>
> static struct ErrorLookupTable MyTable[ TableSize ] =
> {
> { 45, "Not enough memory" },
> { -66, "File I/O error" },
> { -2565, "Device locked" },
> { 32727, "Device not found" },
> { -32727, "Device not found" },
> { 65534, "Unknown device error" }
> //... LOTS more
> };
>
> So I have implemented some prototype code based on this and it does
> work to do lookups. But what I was wondering is how portable is
> this? Is this legal C as defined by the standard to initialize an
> array of structures in this way? Is this going to work across
> platforms? I hope you see my dilemma the code appears to work on my
> system. But I don't know if it's guaranteed to work everywhere and how
> portable the library functions I'm working on will be because of it?

If you provided the errors in sorted order, you could use bsearch to
do the lookup.
I guess that a hash table will work perfectly well and be the best
solution for your problem.
If you don't want to use a hash table, then provided the data in
sorted order or use a skiplist.

P.S.
Besides the good suggestions to use sizeof, I would also suggest
making everything const.

Guest

Thanks everyone!
I am going to implement it with the suggestions by Ulrich and others
especially the use of const and use a sizeof for finding the table
size which I really like. Sorting I also see as the table continues
to grow. Obviously you guys have seen this type of problem before.
Thanks alot!

Guest

Thanks everyone!
I am going to implement it with the suggestions by Ulrich and others
especially the use of const and use a sizeof for finding the table
size which I really like. Sorting I also see as the table continues
to grow. Obviously you guys have seen this type of problem before.
Thanks alot!

wrote:
>
> What I am trying to do is implement a associative array to lookup
> data based on a key in C. This might simple to do in C++ or Perl
> but I'm restricted to use C. The size of the data for the table
> isn't too large but too large to structure the code as a clean
> chain of if/else or similar logic. Also, the lookup key values
> can be large, unpredictable and not well dispersed which doesn't
> suit itself to a hash table.

Take a look at what hashlib can do for you. Purely standard C, and
released under GPL. You don't have to worry about table size. See:

Share This Page

Welcome to The Coding Forums!

Welcome to the Coding Forums, the place to chat about anything related to programming and coding languages.

Please join our friendly community by clicking the button below - it only takes a few seconds and is totally free. You'll be able to ask questions about coding or chat with the community and help others.
Sign up now!